Temperature maintaining apparatus and temperature control apparatus and method therefor

ABSTRACT

A method and apparatus are provided for maintaining a temperature between a first value and a second value. The method and apparatus utilize a heat transfer device capable of cycling through a series of cycles, the device being capable of changing the temperature from the second value to the first value when operational. During each cycle the device transfers heat for a first period of time when the temperature reaches the second volume until the first value is reached and thereafter the device stops transferring heat. The first period of time is compared to a desired cycle-on period of time during each cycle and the second temperature value then is adjusted for a subsequent cycle of the device so the first period of time when the device transfers heat approaches the desired cycle-on period of time. There may also be provided temperature controls for a liquid heater having a jacket containing the liquid. The jacket has an inner heated wall and an outer wall spaced apart from the inner wall. The controls include a temperature sensor capable of fitting to the jacket so that the sensor contacts the inner wall and the liquid. A control module monitors temperature sensed by the temperature sensor. The control module causes the heater to stop heating when the temperature is heated by the heater to a first value and causes the heater to heat when the liquid cools to a second value lower than the first value. The control module senses overheating when temperature sensed by the sensor increases to a third value, higher than the first value, when the heater is not heating, and then causes the heater to shut down.

BACKGROUND OF THE INVENTION

This invention relates to temperature maintaining apparatuses such astransit vehicle heaters and temperature control apparatuses and methodstherefor.

Various temperature maintaining apparatuses, particularly heaters,operate in a cyclic fashion. When the temperature reaches a certainthreshold, such apparatuses operate for a period of time until thetemperature is brought to a certain level and thereafter the apparatusbecomes nonoperational until the threshold is reached again. Forexample, in a heater the temperature may be the temperature of a fluidand the heater operates when the temperature of the fluid drops to a setlower value. The heater continues to operate until the temperature israised to a higher level at which time the heater switches off until thetemperature again drops to the lower level.

One specific type of heater is a transit vehicle heater which operatesto heat the engine coolant. The temperature of the coolant is sensed byat least one temperature sensor and the heater, typically operating ondiesel fuel, is fired until the temperature of the coolant is raised toa higher level. At this point the heater switches off.

This cycling on and off between preset lower and higher temperaturesproduces less than satisfactory performance under some conditions.People in a transit vehicle, for example, may perceive that thetemperature of the air in the vehicle drops to an uncomfortable levelwhen the heater cycles on and off between preset lower and uppertemperatures. This is because too much time may occur near the bottom ofthe cycle when the temperature is low and/or because the meantemperature is reduced below optimum levels. Standard on/off temperaturecontrols or thermostats are not capable of adjusting themselves to sucha situation.

Transit vehicle heaters, as well as some other types of heaters forliquids, conventionally employ two different temperature sensors. One ofthe sensors contacts the liquid, typically a water/anti-freeze mixturein the case of a transit vehicle heater, to determine the temperature ofthe liquid in order to cycle the heater on and off as required tomaintain the temperature of the liquid. A second sensor contacts theinner wall of the jacket to sense an overheat condition. The need fortwo different sensors increases the cost and complexity of the units.

Transit vehicle heaters, and other types of heaters often must beinstalled in different configurations. For example, in the case oftransit vehicle heaters, the direction of flow of liquid through theheaters sometimes must be reversed according to the plumbingrequirements of a particular vehicle. Conventional transit vehicleheaters do not readily adapt to a reversal of liquid flow because oftemperature sensors placed adjacent the fluid inlet and fluid outlet.The heater controls are adversely affected when the fluid flow isreversed because of the effect this has on the readings of the sensors.

Also some conventional heaters require two sensors adjacent the fluidinlet and fluid output respectively and cannot operate with a singlesensor or with one of the sensors disabled. The controls have a lack offlexibility in this respect.

It is an object of the invention to provide an improved heat transferdevice, such as a transit vehicle heater, as well as controls thereforand a method of maintaining a temperature, which are capable of adaptingto different temperature conditions to maintain a more constant meantemperature compared to a simple system where the device cycles on andoff at fixed temperatures.

It is another object of the invention to provide an improved heater forliquids, a method of controlling temperatures for liquids and atemperature control apparatus where a single sensor can act both tomeasure temperatures of the liquids to determine when the heater cycleson and off, and also acts as an overheat sensor for the heater.

It is a further object of the invention to provide an improved heaterfor liquids, a method of controlling such heaters and control systemsfor such heaters which are adaptable to varying installationrequirements, such as reversing the flow of liquids through the heaters.

It is still further object of the invention to provide an improvedheater for liquids, a method of controlling such heaters and a controlsystems for such heaters which can operate with two temperature sensors,with one of the sensors removed or with one of the sensors disabled.

SUMMARY OF THE INVENTION

There is provided, according to a first aspect of the invention, amethod of maintaining a temperature between a first temperature valueand a second, variable temperature value, the method utilizing a heattransfer device capable of cycling through a series of cycles. Thedevice is capable of changing the temperature from the second value tothe first value when operational. During each said cycle the devicetransfers heat for a first period of time when the temperature reachesthe second value until the first value is reached and thereafter thedevice ceases to transfer heat. The first period of time is compared toa desired cycle-on period of time during each said cycle and said secondvalue then is adjusted for a subsequent cycle of the device so saidfirst period of time when the device transfers heat approaches saiddesired cycle-on period of time.

There is provided, according to a second aspect of the invention, anapparatus for maintaining a temperature between a first temperaturevalue and a second temperature value. The apparatus includes a heattransfer device capable of cycling through a series of cycles. Thedevice is capable of changing the temperature from the second value tothe first value when operational. There are controls which control thedevice so during each said cycle the device transfers heat for a firstperiod of time when the temperature reaches the second temperature valueuntil the first value is reached and thereafter the device ceasestransferring heat. The controls compare the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjust the second temperature value for a subsequent cycle of the deviceso said first period of time when the device transfers heat approachessaid desired cycle-on period of time.

There is provided, according to a third aspect of the invention, aheater for a liquid including controls for maintaining a temperature ofthe liquid between a first value and a second lower value, the heaterbeing capable of cycling through a series of cycles. The controlscontrol the heater so during each said cycle the heater heats for afirst period of time when the liquid cools to the second temperaturevalue until the liquid is heated to the first value and thereafter theheater stops heating. The controls compare the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjust the second temperature value for a subsequent cycle of the deviceso said first period of time when the heater heats approaches saiddesired cycle-on period of time.

There is provided, according to a fourth aspect of the invention, acontrol system for a heat transfer device, the control systemmaintaining a temperature between a first value and a variable secondvalue. The heat transfer device is capable of cycling through a seriesof cycles. The device is capable of changing the temperature from thesecond value to the first value. The control system controls the deviceso during each said cycle the device transfers heat for a first periodof time when the temperature reaches the second temperature value untilthe first value is reached and thereafter the device stops transferringheat. The controls compare the first period of time to a desiredcycle-on period of time during each said cycle and said control systemadjusts the second temperature value for a subsequent cycle of thedevice so said first period of time when the device transfers heatapproaches said desired cycle-on period of time.

There is provided, according to a fifth aspect of the invention,temperature controls for a liquid heater having a jacket containing theliquid. The jacket has an inner heated wall and an outer wall spacedapart from the inner wall. The controls include a temperature sensorcapable of fitting to the jacket so that the sensor contacts the innerwall and the liquid. A control module monitors temperature sensed by thetemperature sensor. The control module causes the heater to becomenonoperational when the temperature is heated by the heater to a firstvalue and causes the heater to operate when the liquid cools to a secondvalue lower than the first value. The control module senses overheatingwhen temperature sensed by the sensor increases to a third value, higherthan the first value, when the heater is nonoperational, and then causesthe heater to shut down.

There is provided, according to a sixth aspect of the invention, aheater for a liquid, the heater having a jacket containing the liquid.The jacket has an inner heated wall and an outer wall spaced apart fromthe inner wall. Temperature controls are operatively connected thereto,the controls including a temperature sensor capable of fitting to thejacket so that the sensor contacts the inner wall and the liquid, and acontrol module which monitors temperature sensed by the temperaturesensor. The control module causes the heater to stop heating when thetemperature is heated by the heater to a first temperature value andcauses the heater to heat when the liquid cools to a second temperaturevalue. The control module senses overheating when a temperature sensedby the sensor increases to a third temperature value, higher than thefirst temperature value, when the heater is not heating, and then causesthe heater to shut down.

There is provided, according to a seventh aspect of the invention, amethod of controlling temperature for a liquid heater having a jacketcontaining the liquid. The jacket has an inner heated wall and an outerwall spaced apart from the inner wall. The method includes fitting atemperature sensor to the jacket so that the sensor contacts the innerwall and the liquid, monitoring temperature sensed by the temperaturesensor, causing the heater to stop heating when the temperature isheated by the heater to a first temperature value and causing the heaterto heat when the liquid cools to a second temperature value lower thanthe first value, detecting when temperature sensed by the sensorincreases to a third value, higher than the first value, when the heateris not heating, treating said third value as an overheat condition andthen causing the heater to shut down.

There is provided, according to an eighth aspect of the invention, atemperature control apparatus for a liquid heater having two fittingsfor connecting liquid conduits to the heater, one of the fittings beingeither an inlet or an outlet for liquid, another of the fittings beinganother of the inlet or the outlet for the liquid. The heater has afirst temperature sensor adjacent to said one fitting and a secondtemperature sensor adjacent to said another fitting. The apparatusincludes a memory having values for a first temperature value, where theheater stops heating, a second temperature value where the heatercommences heating, and a third temperature value, higher than the firstvalue, where the heater is shut off. Both sensors are capable of sensingthe first temperature value and the second temperature value and one ofthe sensors also senses the third temperature value. Controls comparetemperature readings from each of the sensors with each of the firsttemperature value, the second temperature value and the thirdtemperature value. The heater has an overheat condition when atemperature exceeds the third temperature value, a high condition when atemperature exceeds the first temperature value, a normal condition whena temperature is between the second temperature value and the firsttemperature value and a low condition when a temperature is less thanthe second temperature value. The apparatus indicates the overheatcondition when one of the sensors indicates a temperature exceeding thethird temperature value, the high condition when either sensor indicatesa temperature exceeding the first temperature value, but neither sensorindicates a temperature exceeding the third temperature value, thenormal condition when either sensor indicates a temperature exceedingthe second temperature value, but neither sensor indicates a temperatureexceeding the third temperature value, and a low condition when bothsensors indicate temperatures less than the second temperature value.

There is provided, according to a ninth aspect of the invention, amethod of controlling temperatures in a liquid heater having twofittings for connecting liquid conduits to the heater, one of thefittings being either an inlet or an outlet for liquid, another of thefittings being another of the inlet or the outlet for the liquid. Theheater has a first temperature sensor adjacent to said one fitting and asecond temperature sensor adjacent to said another fitting. The methodincludes retaining values for a first temperature, where the heaterstops heating, a second temperature where the heater commences heating,and a third temperature where the heater is shut off, both sensors beingcapable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, comparing temperature readings from each of thesensors with each of the second temperature value, the first temperaturevalue and the third temperature value. The heater has an overheatcondition when a temperature exceeds the third temperature value, a highcondition when a temperature exceeds the first temperature value, anormal condition when a temperature is between the second temperaturevalue and the first temperature value and a low condition when atemperature is less than the second temperature value. The overheatcondition is indicated when one of the sensors indicates a temperatureexceeding the third temperature value, the high condition when eithersensor indicates a temperature exceeding the first temperature value,but neither sensor indicates a temperature exceeding the overheattemperature, the normal condition when either sensor indicates atemperature exceeding the second temperature value, but neither sensorindicates a temperature exceeding the first temperature value, and a lowcondition when both sensors indicate temperatures less than the secondtemperature value.

There is provided, according to a tenth aspect of the invention, atemperature control apparatus for a liquid heater having a firsttemperature sensor and a second temperature sensor, the apparatusincluding a memory having values for a first temperature, where theheater stops heating, a second temperature where the heater commencesheating, and a third temperature where the heater is shut off, andcontrols which compare temperature readings from each of the sensorswith each of the first temperature value, the second temperature valueand the third temperature value. Both sensors are capable of sensing thefirst temperature value and the second temperature value and one of thesensors also senses the third temperature value. The heater has anoverheat condition when a temperature exceeds the third temperaturevalue, a high condition when a temperature exceeds the first temperaturevalue, a normal condition when a temperature is between the firsttemperature value and the second temperature value and a low conditionwhen a temperature is less than the second temperature value. Theapparatus indicates the overheat condition when one of the sensorsindicates a temperature exceeding the third temperature value, the highcondition when either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.

There is provided, according to an eleventh aspect of the invention, amethod of controlling temperatures in a liquid heater, the heater havinga first temperature sensor and a second temperature sensor, the methodincluding retaining values for a first temperature value, where theheater stops heating, a second temperature value where the heatercommences heating, and a third temperature value where the heater isshut off. Both sensors are capable of sensing the first temperaturevalue and the second temperature value and one of the sensors alsosenses the third temperature value. Temperature readings from each ofthe sensors are compared with each of the first temperature value, thesecond temperature value and the third temperature value. The heater hasan overheat condition when a temperature exceeds the third temperaturevalue, a high condition when a temperature exceeds the first temperaturevalue, a normal condition when a temperature is between the firsttemperature value and the second temperature value and a low conditionwhen a temperature is less than the second temperature value. Theoverheat condition is indicated when one of the sensors indicates atemperature exceeding the third temperature value, the high conditionwhen either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.

There is provided, according to a twelfth aspect of the invention, amethod of detecting a temperature sensor error in a heater for a liquidincluding a temperature sensor and controls for maintaining atemperature of the liquid between a first value and a second lowervalue, the heater being capable of cycling through a series of cycles.The controls control the heater so during each said cycle the heatercommences heating when the temperature sensor indicates that thetemperature has decreased to the lower value, the controls indicating atemperature sensor error if the temperature indicated by the sensor doesnot increase by a specified amount within a specified time after theheater commences heating.

The invention offers significant advantages compared to the prior art,particularly prior art heaters and other heat transfer devices whichcontrol temperature by switching on and off at fixed temperature values.By comparison the invention allow such a device to adapt to changingambient conditions. If temperatures fall, then a heating device, forexample, tends to operate longer until the temperature reaches a setupper value. This lengthened period of time is sensed by the inventionand the temperature where the heater becomes operational is raised. Thistypically causes the heater to cycle on and off at an increasedfrequency. The effect is to maintain the mean temperature and give anincreased degree of comfort compared with a conventional system.

At the same the invention allows such heat transfer devices to adapt inother ways. For example, a prolonged period of cooling may be sensed andthe heater may be turned on even though the temperature has not fallento a value which would normally trigger operation of the heater. Thisagain maintains a minimum mean temperature and increases the degree ofcomfort.

The invention also permits a single temperature sensor to replace twodifferent temperature sensors normally used for temperature regulationand for sensing an overheat condition. This reduces the number of partsrequired for devices such as transit heaters and simplifies assembly.

The invention offers significant advances and increases adaptability ofvarious heat transfer devices such as heaters and hot liquid transitheaters in particular. For example, where such a heater has twodifferent heat sensors the system can operate without information onwhich sensor is which or in which direction liquid flows through theheater. Moreover, if one sensor is removed, or one becomes faulted, thenthe system can adapt and still operate correctly.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiment so the invention:

FIG. 1 is an exploded isometric view of a transit heater according to anembodiment of the invention;

FIG. 2 is an enlarged front, side isometric view of the burner headassembly thereof;

FIG. 3 is an exploded, isometric view thereof;

FIG. 3a is a phantom view of the control module assembly of FIG. 3 shownin an alternative position rotated 18020 from the position of FIG. 3;

FIG. 4 is a fragmentary, sectional view showing a fragment of the jacketof the heater and a temperature sensor mounted thereon;

FIG. 5 is a flowchart of the first part of a method according to anembodiment of the invention;

FIG. 6 is a continuation of the flowchart of FIG. 5;

FIG. 7 is a continuation of the flowchart of FIG. 6;

FIG. 8 is a continuation of the flowchart of FIG. 7;

FIG. 9 is a chart showing temperature conditions and temperaturedesignations for the heater of FIG. 1;

FIG. 10 is a graph showing how temperature is regulated for aconventional heater as well as showing an overheat condition;

FIG. 11 is a graph showing temperature changes during operation of theheater of FIG. 1; and

FIG. 12 is a graph similar to FIG. 10 showing operation of the heaterduring a lingering drop off.

FIG. 13 is a fragmentary plan view of the burner head housing, partlybroken away to show two air filter mounts and an air filter correctlymounted on one of the mounts;

FIG. 14 is a fragmentary view of a portion of FIG. 13 showing an airfilter incorrectly mounted on one of the mounts;

FIG. 15 is a fragmentary sectional view of the burner head assemblyshowing the air compressor, air filter, air filter mount and airconduits extending between the filter and the compressor;

FIG. 16 is a fragmentary sectional view of the housing showing a rodpreventing an air filter from fitting an incorrect air filter mount;

FIG. 17 is a fragmentary sectional view showing a portion of the housinglocated adjacent to the control module and air filter, in a rotationallynonaligned position;

FIG. 18 is a fragmentary view thereof showing the portion of the housingmoving towards the control module in a rotationally aligned position;

FIG. 19 is a view similar to FIG. 18 showing the portion of the housingfitted onto the control module and against the air filter;

FIG. 20 is a rear, elevational view of the burner head assembly showingthe burner, fuel pump, compressor and associated transformer;

FIG. 21 is a fragmentary sectional view showing two portions of thehousing with the control module fitted therebetween and the circuitboard of the module extending outwardly to connect with two electricalsockets;

FIG. 22 is a fragmentary sectional view showing the Hall effect sensorand magnet on the fan blade assembly for the heater of FIG. 1;

FIG. 23 is a diagram showing the closed loop control system for themotor thereof;

FIGS. 24-27 together comprise a flowchart of the system fordistinguishing overcurrent faults from voltage changes occurring whenthe engine of the vehicle starts;

FIGS. 28 and 29 together comprise a flowchart of the system forselectively soft starting the coolant pump;

FIG. 30 is a graph representing voltage changes when the engine starts;

FIG. 31 is a diagrammatic representation of the heater and coolant pumpof the vehicle; and

FIG. 32 is a flowchart of the system for monitoring sputtering flamesand determining whether or not the heater should be shut off.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and first to FIG. 1, this shows a transitheater 30 according to an embodiment of the invention. The heaterinclude a heat exchanger 32 equipped with a primary temperature sensor34 and a secondary temperature sensor 36. The latter is optional asdescribed below. There are fittings 40 and 42 which serve as inlets andoutlets for liquid, a mixture of water and anti-freeze being the usualliquid for a transit vehicle heater. Mounting brackets 44 and 46 areconnected to the heat exchanger for mounting the heater on a transitvehicle or similar application. A combustion tube 48 fits within theheat exchanger and a burner head assembly 50 fits over the combustiontube. The assembly is secured to the heat exchanger by bolts 52 and 54.

The burner head assembly is shown in better detail in FIGS. 2 and 3. Theassembly includes a burner head assembly housing 51, cylindrical in thisexample, having two portions, a blower housing 56 and a head flangeassembly housing 66. The blower housing 56 has a splash guard 58connected thereto. There is an air intake behind the splash guard. Ablower 60 is mounted on shaft 62 of electrical motor 64. Burner headflange assembly housing 66 is at the end of the burner head assemblyopposite blower housing 56. The motor is mounted on the inside of theflange assembly housing 66 by means of bolts 68. A gear 70 is mounted onthe shaft of the motor and engages another gear 72 which drives acompressor 130 and a fuel pump 131 on the opposite side of the headflange assembly housing as seen in FIG. 20. There is a fitting 74 forconnecting a fuel line to the fuel pump.

Referring again to FIG. 20, there is a burner 132 having a nozzle 134provided with ignitors 136 and 138. The ignitors are connected totransformer 140 which receives electrical current through cable 142connected to internal electrical connector 144. Cable 146 and internalelectrical connector 148 are provided for the fuel solenoid valve.

Control module assembly 80, shown in FIG. 3, fits between the twoportions of the burner head assembly housing 51, namely head flangeassembly housing 66 and blower housing 56. The assembly includes anannular member, or module housing 82, having a central aperture 84 whichfits over the motor. The module housing 82 has a circular outer surface83 which is adjacent to the exterior of the burner head assemblyhousing. As seen in FIG. 21, the housing 56 has an annular recess 160provided with a shoulder 162 while housing 66 has a corresponding recess164 and shoulder 166. The control module has shoulders 168 and 170 whichbutt against the shoulders 162 and 166 to provide a sealing fit betweenthe control module and the burner head assembly housing.

There is a first cylindrical socket or mount 86 for receiving air filter88 as seen in FIG. 3. Likewise there is a second cylindrical socket ormount 87 for receiving the air filter when the control module is rotated180° as shown in FIG. 3a. A circuit board 150 is located within housing82 as shown in FIG. 21. As shown in FIG. 3, a temperature sensor plug 92is received within a socket 98. There is also an auxiliary plug 96 and asocket 94 for receiving a data link plug.

Referring to FIGS. 3 and 3a, the control module assembly 80 can berotated 180° to an alternate position. This is done so that the externalelectrical connectors, such as connector 172, can be rotated to one sideor the other of the housing 51 according to the requirements of thevehicle where the heater is being installed.

As shown in FIG. 15, housing 66 has an air conduit or passageway 180which communicates with air compressor 130. Likewise control modulehousing 82 has an air passageway 182 which aligns with air conduit orpassageway 180 and communicates with the air filter mount 86 for theposition of the control module housing 82 shown in FIG. 3. Thepassageways 182 and 180 therefore allow air to pass from the air filterto the air compressor. The air filter mount 87 also has an airpassageway 184, shown in FIG. 16, which is not used for air in thepositions shown in FIG. 3 and FIG. 16. This is because the aircompressor is on the opposite side of the assembly. It is desirable toprevent maintenance personnel from inadvertently mounting air filter 88in the wrong air filter mount, which would be mount 87 in FIGS. 3 and16. For this purpose a pin 186 is connected to the housing 66 in aposition such as to extend through the air passageway 184 as shown inFIG. 16. If an attempt is made to fit air filter 88 into the wrong mount87, then pin 186 prevents this from happening since the filter contactsthe pin and cannot be inserted properly into the mount as shown in FIG.16.

FIGS. 13 and 14 illustrate how blower cover 56 holds the air filterproperly in position. There is a member or pin 190 inside the blowerhousing 56 which extends towards the air filter when the air filter isproperly positioned as shown. The pin has an outer end 192 which fitswithin flange 194 of the filter and secures the filter in position. Aflange 156 extends between the pin and the rest of the housing apartfrom the outer end of the pin. The pin 190 is properly dimensioned sothat the outer end 192 just contacts the filter when the filter isproperly positioned.

There is a similar pin 196 inside the housing on the side opposite pin190. It may be observed however that pin 196 is longer than pin 190. Asshown in FIG. 14, the outer end 198 of pin 196 contacts the filter 88,if the filter is improperly positioned in mount 87, before the housing56 is properly seated on control module 82. The pin 196 accordinglyprevents the blower housing from fitting properly on the control modulehousing if the filter is improperly positioned.

FIGS. 17-19 illustrate a mechanism for preventing the pin 190 fromdislodging the filter when the blower housing 56 is being mounted on thecontrol module housing 82. There is a key 200 on the blower housing 56and a slot 202 on the module housing 82. There are three more spacedapart similar keys and similar slots on the housings. The keys fitwithin the slots when the housings 56 and 82 are properly aligned andallow the housings to fit together as shown in FIG. 19. If however thehousings are not rotationally aligned, as shown in FIG. 17, then thekeys 200 contact housing 82 and prevent the housings from fittingtogether.

Referring to FIG. 21, the control module housing 82 has an internalcircuit board 150. The control module housing as well as the circuitboard extend from interior 152 of the burner head assembly to exterior154 thereof. The circuit board is connected to a pair of external,electrical connectors 156 and 158, thereby allowing external wiringharnesses to be connected to the control module without requiring wiringharnesses to pass through the housing of the burner assembly. Internallythe circuit board is connected to such internal electrical connectionsas auxiliary plug 96 and socket 98.

Details of the temperature sensor 34, and a fragment of the heatexchanger are shown in FIG. 4. The sensor includes a body 100 with ahexagonal outer portion 102 and a male threaded, hollow inner portion104. The male threaded portion is received in a female threaded socket106 in outer wall 103 of jacket 110 of the heater which surrounds theheat exchanger. The jacket has an inner wall 112 which is exposed toflame 114. Liquid 116, typically water and anti-freeze, is locatedbetween the inner wall and the outer wall. The sensor has a springloaded shank 118 which is biased against the inner wall 112. The sensorhas electrical contacts 120 and 122.

In a conventional heater a sensor, such as sensor 34, would function aseither a temperature sensor for cycling the heater on and off or as anoverheat sensor. With reference to FIG. 10, a normal temperature sensorreads coolant temperatures and turns the burner on when the temperaturereaches a lower level, 65° C. in this example. Such a temperature sensorhas a portion immersed in the coolant. The burner then operates untilthe temperature reaches a first value, an upper limit of 85° C. in thisexample, as illustrated at 124 in FIG. 10. At this point the controlsturn the burner off and the temperature gradually falls to a secondvalue, 65° C. again at 126 and the burner is operated again.

However, heaters conventionally also have an overheat sensor whichsenses, for example, the absence of coolant. If there is no coolant inthe jacket, then the inner wall 112 overheats and the heater is shutdown. An overheat sensor is therefore in contact with the inner wall ofthe jacket. Sensor 34, however, serves both functions, to sensetemperature of the coolant, as well as sensing overheating of theheater. This is done utilizing the structure of the sensor shown as wellas appropriate programming of the control module.

With reference to FIG. 10, the heater is cycled off at points 124 and128 when the maximum temperature is reached. Under overheat conditions,however, the temperature sensed by the sensor continues to increaseafter the burner cycles off as indicated at 130 in FIG. 10. Accordingly,the control module senses an overheat condition and shuts down theheater when the sensor 34 indicates a third value, 90° C. in thisexample, which exceeds the normal maximum temperature, after the heaterhas been cycled off. Thus a single temperature sensor can fulfill bothfunctions described above.

The heater is also adaptable to changing ambient conditions. A normalon/off temperature control, as shown in FIG. 10, may lead to problemswhen temperatures drop. Referring to FIG. 11, the system may aim atproviding a mean temperature of 75° C. by cycling on the heater when thecoolant reaches a lower temperature of 65° C. The heater then cycles onuntil the higher temperature of 85° C. (also referred to as the firsttemperature value herein) is reached, at which time the heater cyclesoff. However, at lower ambient temperatures it takes longer for theheater to heat the coolant to the higher temperature. For example, thetime increases from T1 to T2 as a result of cooling conditions. Theeffect of this is to lower the mean temperature below 75° C. This maymean that the temperature inside a bus, for example, may be too cool forthe occupants.

The invention overcomes this problem by monitoring the time T1 or T2which it takes for the heater to heat the coolant to the highertemperature. When this time increases, the control module increases thetemperature where the heater cycles on (also referred to as the secondtemperature value herein). For example, in FIG. 1 the cycle-ontemperature is increased to 68° C. since the controls determined that T2exceeds T1. The cycle-on temperature is also lowered if the timeinterval to heat the coolant shortens. The cycle-on temperature isvaried in this example between specific limits, the lower limit being65° C. and the upper limit being 78° C. which is a fixed amount, ΔT,below the maximum temperature. The programming of the controls is setout in detail in FIGS. 5-8.

The algorithm can only modify the temperature within the standard rangeof 65° C. to 85° C. The cycle-off temperature is never adjusted. Thisensures that in the worst-case scenario the heater would just revertback to the standard control method. The algorithm updates the cycle-ontemperature once every cooling curve. This ensures that the heater willrapidly adapt to any changes in the parameters of the heating system. Byusing the heat time to calculate the new cycle-on temperature, everyparameter of the heating system is taken into consideration.

Details of the algorithm follow:

Adjusting Cycle-on Temperature

a) The current cycle-on temperature threshold is adjusted at the end ofeach heating interval (i.e. on entry to purge state)

i) based on the formula:

New-current cycle-on temperature threshold=(1−(target heat time/actualheat time))* maximum temperature change+current cycle-on temperaturethreshold

(1) target heat time defines the ideal flame-on-time

(2) actual heat time is measured as duration of heating cycle startingwith entry into ignition state and ending with cycle-off event, butqualified by at least one temperature sensor reading above currentcycle-on temperature threshold

(3) maximum temperature changes intended to limit the amount ofadjustment made in one heating cycle

ii) current cycle-on temperature threshold is not permitted to exceed(cycle-off temperature threshold-minimum delta-T), or be less thancycle-on temperature threshold.

Timed Cycle-On

a) a cycle-on event is forced if cooling time since the end of theprevious heating cycle exceeds the current maximum cool time and atleast one temperature sensor is reading less than cycle-off temperaturethreshold-minimum delta-T

i) a current maximum cool time timeout can occur in standby or standbysupplemental states

ii) a current maximum cool time timeout causes the current cycle-ontemperature threshold to be updated with the greater of the twotemperature sensor readings.

b) for each consecutive heating cycle begun due to current maximum cooltime, the value of current maximum cool time is doubled (for use in thenext cycle)

i) in the subsequent heating cycle, the value of current cycle-ontemperature threshold is adjusted per the formula above, as usual.

ii) if, in the current heating cycle, actual heat time exceeds targetheat time, then current maximum cool time reverts to default maximumcool time.

Short Cycle

a) While purge or purge error state, if T1 and T2<current cycle-ontemperature threshold, the system does not wait for completion of purgestate, but immediately cycles-on the heater, i.e. abandons purge state,and skips standby state. If the heater is operating in supplementalmode, it also skips the pre-run state.

Initial Conditions

a) at power-up, the value of current cycle-on temperature thresholdreverts to the value of cycle-on temperature threshold, and the value ofcurrent maximum cool time reverts to the value of default maximum cooltime. Also, the cooling time timer is reset.

b) At switch-off, the value of current maximum cool time reverts to thevalue of default maximum cool time. Also the cooling time timer isreset.

As described above, the heater can operate with a single temperaturesensor 34 as shown in FIG. 1. Optionally however a second sensor 36 maybe used for some applications. Typically one sensor is adjacent the hotend of the heater, sensor 34 in this example, and another sensor isadjacent the cool end of the heater, sensor 36 in this example. Asdescribed above, both sensors are coolant temperature sensors, butsensor 34 in addition acts as an overheat sensor. It is desirable tohave the system adapt so that the flow of coolant through the heater cantravel from either fitting 40 to fitting 42 or from fitting 42 tofitting 40. The choice depends upon the plumbing requirements of aparticular installation, for example. In such a system with two sensorsis also desirable to have the system operate even if one sensor isremoved or if one sensor becomes dysfunctional.

FIG. 9 is a chart which sets out various temperature designations andranges of temperatures employed by the invention. The heater normallycycles within the Normal range of temperatures between the Current CycleOn Threshold and the Cycle Off Threshold. The Cycle Off Threshold is afixed first value, 85° C. for the example of FIG. 11. On the other hand,the Current Cycle On Threshold, or cycle-on temperature, varies betweena second value, the Minimum Cycle On Threshold, 65° C. in the case ofFIG. 11, to the Maximum Cycle On Threshold, namely 78° C. in the case ofFIG. 11. There is also a third value, an Overheat Threshold which is,for example 90° C. in FIG. 10, which, when sensed, results in shuttingthe heater down. In addition there is the Open Threshold. When thecontrol module receives a voltage reading from a sensor equivalent to atemperature lower than the Open Threshold, then this indicates an opencircuit and that the sensor has faulted. Likewise there is a ShortThreshold which, when exceeded, indicates that the sensor has shorted.

A temperature within the range between the Current Cycle On Thresholdand the Cycle Off Threshold is considered to be within the Normal range.Temperatures below the Current Cycle On Threshold and the Open Thresholdare considered in the Low temperature range. Temperatures below the OpenThreshold indicate a Faulted condition.

Temperatures above the Maximum Cycle On Threshold and below the ShortThreshold are considered within the Warm range. However temperaturesabove the Cycle Off Threshold and below the Short Threshold areconsidered in the High range. Temperatures above the Overheat Threshold,but below the Short Threshold are in the Overheat range. Finallytemperatures above the Short Threshold show a Faulted condition.

The heater may be configured to expect one or two temperature sensors.The temperature sensor may be connected to either sensor connection onthe control module. When the system is configured to expect twotemperature sensors, the coolant flow through the heat exchanger may benon-specific. This object is achieved by combining the values of the twosensors into a single overall status according to the following table:

TABLE 1 Temp Sensor Temperature Sensor 1 2 F OH H W N L F F F F F F F OHF OH OH OH OH OH H F OH H H H H W F OH H W W W N F OH H W N N L F OH H WN L

In the above table the temperature ranges in the upper row are thosesensed by sensor 1. The temperature ranges in the left-hand column arethose sensed by sensor 2. F indicates a temperature in the Faultedrange, OH a temperature in the Overheat range, H a temperature in theHigh range, W a temperature in the Warm range, N a temperature in theNormal range and L a temperature in the Low range.

Alternatively there may be conditions when only one sensor is required,but actually two are present. In this case the control module does notattempt to determine which one is present. The table below defines theoverall condition assuming that the absent sensor appears Faulted/open.When there is only one sensor present or required, and if it is faulted,then the control module does not know which sensor is faulted, so faultson both sensors are generated even though there is only one.

TABLE 2 Temp Sensor Temperature Sensor 1 2 F OH H W N L F F OH H W N LOH OH OH OH OH OH OH H H OH H H H H W W OH H W W W N N OH H W N N L L OHH W N L

For prior art utilizing an NTC thermistor, it is possible the smallamount of moisture or corrosion across the sensor leads can simulate acold temperature reading. This may cause the heater to fire with a falselow reading, and may allow the heater to operate indefinitely if thereading does not change.

An algorithm is used to detect a temperature sensor that is notconsidered open or shorted, but stuck at some level. It is considered aDelta-T fault if at least one temperature level does not increase by aminimum Delta-T (3° C. in this example) from the time the heater entersthe Ignition state until it has been in the Run/Reignition states for aDelta-T check time (60 seconds in this example). If either temperatureincreases by the minimum Delta-T or more, then there is no Delta-Tfault. Otherwise a Delta-T fault is indicated for each sensor (which isnot faulted open/short) whose value was less than a maximum initialtemperture (25° C. in this example) at cycle-on time.

A further application of this algorithm is to operate it at all timesthat the burner is firing, and evaluate the temperature reading againsttypically anticipated values.

While the controls and methods described above are particularly adaptedfor transit vehicle heaters and other vehicle heaters, they may also beuseful, with some alterations, for use with other heaters or other heattransfer devices such as furnaces or air conditioners. Air conditionerstypically cycle on and off between fixed higher, cycle-on temperaturesand fixed, cycle-off temperatures. The invention can be utilized forexample to vary the cycle-on temperature to maintain a desirable averagetemperature.

Referring back to FIG. 3, the blower 60 includes a fan blade assembly 61which is disc-shaped and has a plurality of blades 63. As shown in FIG.22, there is a magnet 65 mounted on the assembly 61 between a pair ofprojections 67 and 69. CPU board housing 90, shown in FIG. 3 and FIG.22, houses circuit board 91 which has a Hall effect sensor 93 on the endthereof which faces the fan blade assembly. The Hall effect sensor isaligned with the magnet so that the magnet passes the Hall effect sensoron each rotation of the fan blade assembly and, accordingly, on eachrotation of the motor 64. The Hall effect sensor therefore acts as aspeed sensor which is responsive to rotational speeds of the fan.

A programmable control module mounted on the circuit board 91 isoperatively connected to the Hall effect sensor and includes a closedloop feedback control for the motor as shown in FIG. 23. A desired motorspeed is inputted at 250 and the processor compares this value at 252 tothe momentarily measured value 254 as sensed by the sensor. Thecalculated error 256 is inputted into control module 258 which drivesprocess 260 with the value 262 to change its output 264.

The use of speed control provides significant advantages over earliervehicle heaters where speed control has not been used. Accordingly,motor speed varied as much as 50 percent depending upon the voltagesupplied to the heater. The addition of speed control means that thespeed of the motor is independent of voltage and the output of theheater can be regulated by selecting a particular motor speed which willgive the heater the required amount of fuel and air for the designatedoutput. Furthermore, the heater can be a single speed heater or avariable speed heater which accordingly can change the output. Forexample, the output could be increased initially to heat up a vehicleand then decreased to maintain a steady temperature.

The heater 30 has a backup speed control system in case of failure ofthe system described above including, for example, failure of the Halleffect sensor or physical dislocation of the magnet. The control moduleincludes a lookup table. It looks up the voltage supplied to the heaterin the lookup table and uses pulse width modulation to yield the desiredrotational speed for the motor. For example, Table 3 below shows thatfor desired rotational speed of 3600 rpm, the required PWM at a supplyvoltage of 12 volts is 85 percent.

TABLE 3 Volts PWM 9 100 10 95 11 90 12 85 13 83 14 80 15 78 16 76 17 7418 72 18 70 19 68 20 66 21 64 22 62 23 61 24 60 25 59 26 58 27 57 28 5629 55 30 54

Pulse width modulation is used to reduce the speed to the requiredamount even if the voltage is higher. During operation of the Halleffect sensor, the table is constantly updated to indicate the amount ofpulse width modulation required to yield the correct rotational speedfor a particular voltage applied to the heater. If the Hall effectsensor fails, then speed control is maintained utilizing this table.Effectively the control module strips off voltages above 9 volts in theabove example.

The use of the speed control system utilizing pulse width modulationallows the heater to be used for electrical systems having differentvoltages. In this example the heater 30 runs at 9 V, but can be utilizedon 12 V or 24 V systems. The speed controller strips off the voltagesabove 9 V as mentioned above. Also the output of the heater can beincreased or decreased by increasing or decreasing rotational speed ofthe motor. A few other modifications are necessary including changingthe nozzle 134. Different motors are not required for different heateroutputs, but rather a single motor can be used for different heatingcapacities unlike the prior art. This reduces the number of componentswhich must be ordered and stored. A personal computer utilizingappropriate software can be connected to a port on the heater and usedto change the speed of the motor has desired.

Before the control module commences the combustion process, it exercisesselective heater components to allow a service technician to directlyobserve and verify operation of these loads. This facilitatestroubleshooting and eliminates the requirement for special test tools.

In the heater 30, the status of the flame sensor 149, shown in FIG. 20,is mirrored by an indicator light 161 shown in FIG. 2. This removes theneed for a sight glass to allow an operator or technician to view thecombustion area for the presence or absence of the flame. As describedabove, the flame sensor is integrated into the control module. Theoperation of the flame sensor should be independently verified so thatthe entire control module is not replaced for what might be a combustionrelated problem. With the heater switched off, but with power suppliedto the heater, the burner head is removed from the heater assembly and aflashlight is directed onto the flame sensor. If the indicator lightturns on, then the flame sensor functionality is confirmed. This caneliminate the flame sensor as a potential problem when troubleshooting.

FIG. 31 shows a coolant pump 151 which is connected to heater 30 bycoolant conduit 153 and to the cooling system of engine 155. A cable 157connects the pump to the heater and supplies the pump with power whenoperation of the pump is required.

The coolant pump may have a current limit which is less than the inrushcurrent encountered when the coolant pump motor is started. For example,the current limit for the motor may be 10 amps, but the inrush currentmay be 20 amps. A soft start may be employed so as to reduce the currentsupplied to the motor when the motor starts. In the case where a largepump is utilized, it must be indirectly driven through the use of arelay. However, soft starts may cause chatter in the relay. This causesthe relay to eventually fail. Accordingly, the software for the controlmodule uses a special procedure to turn on the coolant pump output. Thisis shown in detail in the flowchart of FIGS. 28-29.

The software initially turns on the coolant pump output. If the loadcurrent exceeds a preset maximum, the hardware turns off the output andasserts its shut-off line. One millisecond after turning on the output,the software checks the shut-off line. If the coolant pump is stillshutting off after two seconds, then the Control module declares acoolant pump fault. At any of the shut-off checks on one millisecondintervals, if the shut-off line is not asserted, then the Control modulesets up a one second timer. If there are no further shut-offs by thetime that the timer expires, then the Control module declares the pumpsuccessfully started and any subsequent shut-offs are declared coolantpump faults. However if a shut-off does occur before the one secondtimer expires, then the Control module resumes its one millisecond checksequence (it is still within two seconds of the start of the softwareprocedure). This procedure essentially results in a 1 kHz variable dutycycle pulse width modulation (PWM) that lasts no longer than twoseconds, with successful starts known to have been running for at leastone second without faltering.

Using this approach large loads with an inrush current exceeding thepreset maximum will be soft-started, thus protecting the control modulefrom low-voltage transients, and protecting the load fromdemagnetization (only if it is a motor). Loads with inrush currentsbelow the preset maximum will be hard-started. When using a relay todrive a large coolant pump, this prevents relay chatter and prolongsrelay life.

Essentially this means that a soft start is selectively used if thecurrent is above a certain level and hard start is used if the currentis below this level to extend relay life. The soft start turns on andoff rapidly like a pulse width modulation.

During starting of the vehicle engine, the voltage supply to the heaterdrops as the engine is cranked by the starter motor. The voltage thenjumps when the alternator becomes operational. This voltage jump mayshow a false high current fault and consequently problems for theoperator. The invention addresses this problem by looking for rapidvoltage changes when an overcurrent condition occurs.

The shown in FIG. 29 when the vehicle is started and the Control moduleis operating, the Control module sees a drop in supply voltage duringengine cranking, perhaps for several seconds. The motor speed controlwill probably increase the motor duty cycle to compensate. This drop insupply voltage is followed by a sudden increase in supply voltage whenthe alternator becomes operational. Such a rapid increase in supplyvoltage could result in motor and/or coolant pump overcurrent faults.

The means of overcoming this problem is shown in the flowchart of FIGS.24-27. The Control module continuously keeps track of whether there hasbeen a large change in supply voltage. Supply voltage level was measured10 times per second with the last 8 samples retained. As each new sampleis obtained, the Control module compares it with the sample taken 0.7seconds ago. If the voltage rose by more than 1 volt, a 1 bit is shiftedinto a 32-bit shift register, allowing up to 3.2 seconds of history.Otherwise, once per 0.1 seconds, a zero is shifted in. If the voltagefell by more than 1 volt, a 1 bit is shifted into a separate 32-bitshift register, allowing up to 32 seconds of history. Once per second azero is shifted in.

When a coolant pump or motor (peak or average) overcurrent fault occurs,the Control module checks to see if any rise events occurred in the last2 seconds, or any fall events occurred in the last 30 seconds. If so,then the apparent coolant pump or motor current fault is declared a dVfault and essentially ignored. The fault is logged using a new codeindicating rising and/or falling supply voltage. The Finite StateMachine logic which runs the control module proceeds to a Purge Errorstate. The Error Count does not increment and the indicator light doesnot blink. It will be readily understood that the values given above areby way of example and would be altered in different embodiments.

With reference to the flowchart of FIG. 32, the invention includesprovision for taking care of a sputtering flame caused, for example, byair bubbles in the fuel. These air bubbles can cause the flame tosputter or go out. If this occurs, then it is necessary to restartcombustion. The invention utilizes two timers, a 15 second flame-ontimer and a 10 second flame-out timer. While operating, if the flameextinguishers, then the flame sensor indicates that there is no flameand this turns on the ignition. This attempts to reignite the flame.While the heater is running normally (i.e. with a flame) a flame-ontimer runs as long as the heater is in the Run state. This timer isfrozen in the Reignition state. The Flame-on timer and the Flame-outtimer are reset when the Flame-on timer times out after being in Runstate for 15 seconds.

A flame-out timer keeps track of how long the flame has been out. Afterbeing out for 10 seconds, a flame-out fault is declared. If the flamereignites, the unit returns to the Run state. The Flame-out timer is notcleared when the unit returns to the Run state rather the Flame-outtimer is frozen in case the flame goes out again right away, and thesystem returns to the Reignition state again.

The 10 seconds for the Flame-out timer and 15 seconds for the Flame-ontimer are significant. The system tolerates 10 seconds/25 seconds withthe flame out. In other words, the flame may be out 40 percent of thetime and the heater continues to run. Any more, then the heater willstop since this usually indicates a fault such as a leaking fitting.

The heater described above and shown in the drawings is an auxiliaryheater for buses and trucks. Engine coolant is pumped through the heatexchanger which surrounds the combustion chamber. The heater burnsvehicle fuel. There are two manually operated switch inputs: a maintoggle/rocker switch; and a pre-heat momentary push-button switch. Theunit also has two inputs that come from the engine or an electronicengine controller. There is a coolant pump input which allows the enginecontroller to turn the unit's coolant pump on when the unit is otherwiseoff. There is also a supplemental input which directs the Control moduleto produce supplemental heat for the passenger compartment.

The unit has control over four primary devices. The first is the blowermotor which blows air through the combustion chamber and provide suctionfor the fuel. The air movement provides oxygen for combustion, removesexhaust gases and cools the chamber after the flame is put out. Thesecond is the coolant pump that helps move liquid engine coolant betweenthe input and output ports of the heat exchanger. The third is asolenoid that controls a fuel valve. The fourth is the spark ignitorused to start the fuel burning. The ignitor is turned off after the fuelstarts to burn. Normally the flame continues until the supply of fuel isswitched off.

The unit has additional inputs to sense the presence of a flame, measurecoolant temperature, and detect over/under voltage and other faults, andhas additional outputs for an indicator lamp and to powerauxiliary/accessory devices. Non-volatile memory is used to record hoursmeters and keep an event/fault log. The unit has a serial diagnosticport which allows a remote PC to access/control unit operation.

There is a heating cycle which is defined as a sequence of automaticoperations by the Control module beginning with detecting temperaturebelow the cycle on threshold and starting combustion, and ending withdetecting temperature above the cycle off threshold and extinguish incombustion.

Once a heating cycle starts, there may be fuel and/or hot exhaust gasesin the combustion chamber. When the heating cycle ends, whether or notit terminates successfully, the Control module continues to run theblower motor for a period of time in order to clear out in cool down thecombustion chamber. This process is known as purging.

The Control module of the preferred embodiment has an RS232communication port over which it can interact with a diagnostic programrunning on the standard PC.

Many aspects of the Control module operation are governed by parametersaccessible and modifiable via the datalink.

The behavior of the heater is specified by a finite state machine with16 defined states in the preferred embodiment. However the unit isconsidered to be operating in one of four modes discussed below.

The Normal Mode is the primary mode for the unit. Operation during thismode depends on the state of the main switch. When the main switch ison, the coolant pump runs continuously, and the burners turned on/offaccording to temperature set points (i.e. similar to a thermostat for ahouse furnace). When the main switch is off, the burners stays off, butthe coolant pump runs whenever requested by the engine controller.

Supplemental mode is similar to Normal mode (with the Main switch on),except that the coolant pump does not run continuously. In Supplementalmode, the coolant pump only runs while the burner is on or whenrequested by the engine controller. This mode is selected by turning onthe Supplemental input (while keeping the Main switch off). Supplementalmode is canceled when the Main switch is turned on.

Preheat mode is similar to the Normal mode (with the main switch on),except that it automatically shuts off after 90 minutes. Preheat mode isentered when the operator presses the Preheat pushbutton switchmomentarily. The switch is only honored when both main and supplementalinputs are off. Preheat mode is canceled when either of these otherswitches is turned on.

There are three levels of severity of failure conditions which mayoccur. The first level is noncritical. Some aspect of the unit hasfailed, but it still can perform its basic heater function and thecurrent heating cycle is allowed to continue.

The second level is critical. Here the unit cannot continue the currentheating cycle any longer. The cycle is terminated, but another(automatic) heating cycle is permitted regardless of how many differentcritical faults have occurred within the cycle.

If two consecutive heating cycles are terminated in this manner, it isconsidered catastrophic. Here the unit cannot automatically initiate anymore heating cycles. Operator intervention is required. For example, andoverheat fault is considered catastrophic.

Once a fault has been recognized, and acted upon, the control modulemust consider the fault condition to be cleared before acting on itagain. This prevents a single event from triggering repeated logentries. The control module remembers which faults are currently activeand resets this memory under the following conditions:

For critical and catastrophic faults, all such faults are reset upon atransition from a class B state to a class A state. Purge, purge error,purge shutdown, purge off, shutdown and shutdown override, all discussedbelow, are class B states. All others are class A states.

For noncritical faults, all such faults are reset as above for criticalfaults and also on entry to the off state and on exit from the purgestate. Again these states are discussed below.

The operation of the heater will now be explained with reference to thevarious operational states thereof. The operation of the heater isspecified by a finite state machine (FSM) with the following states. Ingeneral all of the states monitor switch inputs for mode changes, exitPreheat mode when time expires and check for faults on given outputs andinputs of interest.

Powered Off—This represents the state of the electronic control modulewhen it is powered off. When the power is turned on, the heater normallyenters the (heater) Off state.

Off—The heater is off in this state. The electronic control modulehowever only stops running when the power supply to the control moduleis disconnected. All operator switches are off and the unit isconsidered to be in Normal mode awaiting operator or engine controlmodule input.

The unit is intended to be powered, normally by the vehicle battery, atall times. Therefore the heater has a low-power sleep mode while in thisstate. Any manual switch operation, request from the engine controlleror diagnostic port connection will wake it up.

While in the Off state, the indicator light is used to show the presenceor absence of the flame as detected by the flame sensor. This is topermit a service technician to verify the functionality of the flamesensor.

Ignition Check—This state occurs just after the heater has been switchedon while in the Off state using the Main switch. The unit turns theignitor on for five seconds (Ignition Check timeout parameter), allowingthe service technician to verify ignitor functionality. Ignitor faultsare not checked during this period. The state will terminateprematurely, and the unit returns to the Off state if the Main switch isturned off, otherwise the next state is Standby.

Standby—The unit in this state has been switched on by one of theoperator switches, but the burner is not on. The unit monitors coolanttemperature and initiates the process to turn on the burner when thetemperature drops below a lower threshold. The coolant pump is runningcontinuously in this state. The state may occur in any of the threeoperating modes. However the only way it can occur in Supplemental modeis if the engine controller requests that the coolant pump run.

Standby Supp—This state is only for Supplemental mode. It is similar tothe Standby state except that the coolant pump is off. The enginecontroller does not request the coolant pump to run. If the enginecontroller does request the coolant pump, then the unit changes toStandby stage. If the burner needs to be turned on, the unit goes to thePrerun state.

Prerun—This state occurs only for the Supplemental mode. The purpose ofthis state is to run the coolant pump for thirty seconds. It then checksif the temperature sensed still requires the burner to be turned on.This is because the coolant pump has been off and the unit may not bereading the true coolant temperature. The heat from the engine itselfmay be sufficient and there may be no need to turn on the burner.

Precheck—This is the first of a sequence of states the unit goes throughto turn on the burner. Power is applied to the ignition module, butsparking is not enabled. The state lasts about 0.5 seconds, giving theunit time to check for a few types of fault conditions. The checksperformed include:

is the flame already on? (May indicate a faulty flame sensor)

are the temperature sensors okay?

is there an overheat condition (combustion chamber too hot)?

is system voltage within acceptable operating range?

are there ignition module, fuel solenoid or coolant pump faults present?

Preignition—This is the second of a sequence of states turn on theburner. The blower motor is turned on and ignition module sparking isenabled at this point. The fuel valve is kept closed. The state lastsfor about five seconds, giving the unit time to verify motor startup anddetect ignition module faults.

Ignition—This is a third of a sequence of states that the unit goesthrough to turn on the burner. The fuel valve is opened at this point.The objective is to ignite the fuel. The state lasts about thirtyseconds. During this interval, in addition to the usual array of faultconditions, the unit monitors whether the flame is out. At the end ofthis interval, if the flame had not been on sufficiently long enough(see Start Criteria parameter), then the sequence is aborted because theburner failed to start.

Run—This is the final state in the sequence the unit goes through toturn on the burner. Ignition module sparking is turned off at thispoint. Fuel should continue to burn. The unit remains in the state untilcoolant temperature reaches the upper threshold, the Main orSupplemental switch is turned off, or some critical fault is detected.Should the flame go out, the unit attempts reignition by going to theReignition state. When it is time to terminate the current heatingcycle, the unit goes into one of the Purge states to clear thecombustion chamber of exhaust gases and cool it down.

Reignition—When the flame goes out during the Run state, the unitattempts to reignite it in this state. Ignition module sparking isre-enabled. The state lasts for up to ten seconds or until a flame issensed again. A flame-out timeout timer keeps track of how long theflame has been out. After being out for ten seconds, a flame-out faultis registered. If the flame reignites, the unit returns to the Run statewith sparking off. The Reignition flame-out timeout timer is not clearedwhen the unit returns to the Run state. Rather the Reignition flame-outtimeout timer is frozen in case the flame goes out again right away, andthe heater returns to the Reignition state again. The second timer knownas the Reignition flame timeout timer runs only in Run state (and isfrozen while in Reignition state). The reignition flame timeout timer isreset when the Reignition flame timeout timer times out (after being inthe Run state 15 seconds), the Reignition flame timeout timer alsorestarts.

Purge/Purge Off/Purge Error/Purge Shut down—After the burner is turnedoff at the end of the heating cycle, the combustion chamber is clearedof exhaust gases and cooled by running the blower motor for about 2minutes. There are four variations of the Purge state, depending on howthe cycle ended and what the state of the unit will be after the purgingis completed.

Purge—normal termination of heating cycle because upper temperaturethreshold was reached. Unit remains in current operating mode with nextstate being Standby.

Purge Off—normal termination of heating cycle because operator switchedoff the unit or Pre-heat interval timed out. After purge period expires,unit goes to Off state in Normal mode.

Purge Error—heating cycle terminated due to a critical failure. Whilepurging, an error code is displayed on the indicator, but after thepurge period expires. The unit remains in its current operation modewith the next state being Standby.

Purge Shutdown—heating cycle terminated due to a catastrophic failure.An error code is displayed on the indicator, and after the purge periodexpires, the next state is Shutdown (the error code continues to bedisplayed).

When the blower motor is on during a purge state, it is important thatthe blower be kept running if possible to adequately cool the burner andvent exhaust gases and unburnt fuel. About one second after a blowermotor fault, the motor output is retried. Blower motor PWM graduallyramps up to the target motor speed. This may take several seconds. Thereis one exception to this motor retry while in purge strategy, namely ifthe flame sensor detects a flame (see Purge flame timeout parameter),then the motor is turned off (and not retried) in an attempt toextinguish the flame.

Shutdown—The unit in the state has automatically turned itself off dueto a catastrophic failure. The unit remains in this (or C.P. Override)state until operator presence is indicated by switch operation. If Mainand/or Supplemental switches were on at the time of failure, theoperator must switch them both off. If the Preheat mode was active atthe time of failure (Main and Supplemental switches must have been off),the operator must turn the Main or Supplemental switch on (This does notengage the heater in the corresponding mode, rather the unit stays inShutdown state, but no longer considers itself in Reheat mode.) and offagain. The unit then returns to Off state in Normal mode.

C.P. Override (Shutdown Override)—While the unit is in Shutdown state,the engine controller can still request that the coolant pump run. Thestate is essentially identical to Shutdown except the coolant pump isturned on. When the engine controller removes its request, the unitreturns to Shutdown state. If there is a coolant pump failure, it isretried every 10 seconds.

C.P. Run (Off Override)—While the unit is in Off state, the enginecontroller can still request that the coolant pump run. The state isessentially identical to Off state, except the coolant pump is turnedon. When the engine controller removes its request, the unit returns toOff state. If there is a coolant pump failure, it is retried every 10seconds.

It will be understood by someone skilled in the art that many of thedetails provided above are by way of example only and can be altered ordeleted without departing from the scope of the invention which is to beinterpreted with reference to the following claims.

What is claimed is:
 1. A method of maintaining a temperature between afirst temperature value and a second, variable temperature value, themethod utilizing a heat transfer device capable of cycling through aseries of cycles, the device being capable of changing the temperaturefrom the second value to the first value when operational, during eachsaid cycle the device transferring heat for a first period of time whenthe temperature reaches the second value until the first value isreached and thereafter the device ceasing to transfer heat, said firstperiod of time being compared to a desired cycle-on period of timeduring each said cycle and said second value then being adjusted for asubsequent cycle of the device so said first period of time when thedevice transfers heat approaches said desired cycle-on period of time.2. The method as claimed in claim 1, wherein the device is a heater andthe first value is higher than the second value.
 3. The method asclaimed in claim 2, wherein the heater has a burner, the burner becomingoperational when the second temperature value is reached and becomesnonoperational when the first temperature value is reached.
 4. Themethod as claimed in claim 3, the device being a fluid heater and thefirst value, and the second value being temperatures of a fluid heatedby the heater.
 5. The method as claimed in claim 4, wherein the fluid isan aqueous solution including water and anti-freeze.
 6. The method asclaimed in claim 5, wherein the heater is a transit vehicle heater. 7.The method as claimed in claim 3, wherein the second temperature valueis increased after each cycle where the first period of time exceeds thedesired cycle-on period of time.
 8. The method as claimed in claim 7,wherein the second temperature value is decreased after each cycle wherethe desired cycle-on period of time exceeds the first period of time. 9.The method as claimed in claim 3, wherein the heater becomes operationalfor another cycle after the heater is nonoperational for a preselectedperiod of time, whether or not the second temperature is reached.
 10. Anapparatus for maintaining a temperature between a first temperaturevalue and a second temperature value, the apparatus including a heattransfer device capable of cycling through a series of cycles, thedevice being capable of changing the temperature from the second valueto the first value when operational, and controls which control thedevice so during each said cycle the device transferring heat for afirst period of time when the temperature reaches the second temperaturevalue until the first value is reached and thereafter the device ceasestransferring heat, said controls comparing the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjusting the second temperature value for a subsequent cycle of thedevice so said first period of time when the device transfers heatapproaches said desired cycle-on period of time.
 11. The apparatus asclaimed in claim 10, wherein the device is a heater and the first valueis higher than the second value.
 12. The apparatus as claimed in claim11, wherein the heater has a burner, the burner becoming operationalwhen the second temperature value is reached and becomes nonoperationalwhen the first temperature value is reached.
 13. The apparatus asclaimed in claim 1, the device being a fluid heater and the first valueand the second value being temperatures of a fluid heated by the heater.14. The apparatus as claimed in claim 1, wherein the fluid is an aqueoussolution including water and anti-freeze.
 15. The apparatus as claimedin claim 13, wherein the heater is a transit vehicle heater.
 16. Theapparatus as claimed in claim 13, wherein the controls increase thesecond temperature value after each cycle where the first period of timeexceeds the desired cycle-on period of time.
 17. The apparatus asclaimed in claim 1, wherein the controls decrease the second temperaturevalue after each cycle where the desired cycle-on period of time exceedsthe first period of time.
 18. The apparatus as claimed in claim 13,wherein the controls cause the heater to become operational for anothercycle after the heater is nonoperational for a preselected period oftime, whether or not the second temperature value is reached.
 19. Aheater for a liquid including controls for maintaining a temperature ofthe liquid between a first value and a second lower value, the heaterbeing capable of cycling through a series of cycles, the controlscontrolling the heater so during each said cycle the heater heats for afirst period of time when the liquid cools to the second temperaturevalue until the liquid is heated to the first value and thereafter theheater stops heating, said controls comparing the first period of timeto a desired cycle-on period of time during each said cycle and saidcontrols adjusting the second temperature value for a subsequent cycleof the device so said first period of time when the heater heatsapproaches said desired cycle-on period of time.
 20. The heater asclaimed in claim 19, wherein the heater has a burner, the burnerbecoming operational when the second temperature value is reached andbecomes nonoperational when the first temperature value is reached. 21.The heater as claimed in claim 19, wherein the liquid is an aqueoussolution including water and anti-freeze.
 22. The heater as claimed inclaim 19, wherein the heater is a transit vehicle heater.
 23. The heateras claimed in claim 19, wherein the controls increase the secondtemperature value after each cycle where the first period of timeexceeds the desired cycle-on period of time.
 24. The heater as claimedin claim 19, wherein the controls decrease the second temperature valueafter each cycle where the desired cycle-on period of time exceeds thefirst period of time.
 25. The heater as claimed in claim 19, wherein thecontrols cause the heater to commence heating for another cycle afterthe heater has stopped heating for a preselected period of time, whetheror not the second temperature value is reached.
 26. The heater asclaimed in claim 19, wherein the controls include a temperature sensor.27. The heater as claimed in claim 19, wherein the controls include aprogrammable control module.
 28. A control system for a heat transferdevice, the control system maintaining a temperature between a firstvalue and a variable second value, the heat transfer device beingcapable of cycling through a series of cycles, the device being capableof changing the temperature from the second value to the first value,the control system controlling the device so during each said cycle thedevice transfers heat for a first period of time when the temperaturereaches the second temperature value until the first value is reachedand thereafter the device stops transferring heat, said controlscomparing the first period of time to a desired cycle-on period of timeduring each said cycle and said control system adjusting the secondtemperature value for a subsequent cycle of the device so said firstperiod of time when the device transfers heat approaches said desiredcycle-on period of time.
 29. The control system as claimed in claim 28,wherein the device is a heater and the first value is higher than thesecond value.
 30. The control system as claimed in claim 28, wherein theheater has a burner, the burner becoming operational when the secondtemperature value is reached and becomes nonoperational when the firsttemperature value is reached.
 31. The control system as claimed in claim30, the device being a fluid heater, the first value and the secondvalue being temperatures of a fluid heated by the heater.
 32. Thecontrol system as claimed in claim 28, wherein the heater is a transitvehicle heater.
 33. The control system as claimed in claim 28, whereinthe control system increases the second temperature value after eachcycle where the first period of time exceeds the desired cycle-on periodof time.
 34. The control system s as claimed in claim 28, wherein thecontrol system decreases the second temperature value after each cyclewhere the desired cycle-on period of time exceeds the first period oftime.
 35. The control system as claimed in claim 28, wherein the controlsystem causes the heater to heat for another cycle after the heaterstops heating for a preselected period of time, whether or not thesecond temperature value is reached.
 36. The control system as claimedin claim 28, including a temperature sensor and a programmable controlmodule.